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 Astronomy Sci Advisor PF Gold P: 23,235 For example the energy (per volume) in the CMB is something we can find by raising T to the fourth and multiplying by a = pi2/15 The energy of an average CMB photon is 2.701kT and since the temperature is 1.93E-32 and k=1, the average energy is just 5.2E-32. the energy density aT4 is just 9.13E-128 dividing by the average quantum energy gives the number of CMB photons per unit volume in space 1.7E-96 If we want to see this for a large volume, a cubic mile for instance is E114. So the number of CMB quanta in a cubic mile has to be 1.7E18 Just to mull this over a bit: there are 1.7 billion billion CMB photons in a cubic mile and how much energy have they lost since their emission (called "recombination" or "last scattering", believed to be 300,000 years since bang.) They were emitted at z = 1100, so they have lost all but 1/1100 of their original energy. The current CMB energy density was just calculated to be 9.13E-128. So the density of lost energy is 1100 times that: 1.004E-124, essentially E-124. The energy lost from a cubic mile of CMB is E114 times that or E-10-----two tenths of a joule. As yet no general global energy conservation theorem has been proved in General Relativity. Have to think about this two tenths of a joule---did it go anywhere or was it simply lost?